Linkage disequilibrium is a term used in the study of
population genetics for the non-random association of
alleles at two or more
loci, not necessarily on the same
chromosome. It is not the same as
linkage, which describes the association of two or more loci on a chromosome with limited recombination between them. Linkage disequilibrium describes a situation in which some combinations of alleles or genetic markers occur more or less frequently in a population than would be expected from a random formation of
haplotypes from
alleles based on their frequencies. Non-random associations between polymorphisms at different loci are measured by the degree of linkage disequilibrium (LD). A comparison of different measures is provided by
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(LD): The tendency for two 'alleles' to be present on the same chromosome (positive LD), or not to segregate together (negative LD). As a result, specific alleles at two different loci are found together more or less than expected by chance. The same situation may exist for more than two alleles. Its magnitude is expressed as the delta value and corresponds to the difference between
the expected and the observed haplotype frequency. It can have positive or negative values. LD is decreased by recombination. thus, it decreases every generation of random mating unless some
process opposing the approach to linkage equilibrium. Permanent LD may result from natural selection if some gametic combinations result in higher fitness than other combinations. Link to a lecture on
linkage disequilibrium.
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